MENDELIAN GENETICS

Gregor Johann Mendel(1822-1884)•Austrian Monk •Studied the inheritance of traits in pea plants •Developed the laws of inheritance.•Mendel’s work was ignored until the turn of the 20th century.

Peas •He found that the plants’ offspring retained traits of the parents. • In other words they were passed down! •Called the “ Father of Genetics”

Particulate Inheritance •Mendel states that the physical traits are inherited as “particles” •He didn’t know that the “particles” were actually chromosomes & DNA.

Mendel’s Pea Plant Experiments

Why Peas, Pisum sativum?•Can be grown in a small area•Produce lots of offspring•Produce pure plants when allowed to self-pollinate several generations. •Can be artificially cross-pollinated.

Reproduction in Flowering Plants

• Self-Fertilization can occur within the same flower• Cross-fertilization can occur between two different flowers

Seven Pea Plant Traits Studied by Mendel•Seed shape --- Round (R) or Wrinkled (r)•Seed Color ---- Yellow (Y) or  Green (y)•Pod Shape --- Smooth (S) or wrinkled (s)•Pod Color ---  Green (G) or Yellow (g)•Flower position---Axial (A) or Terminal (a)•Plant Height --- Tall (T) or Short (t)•Flower color --- Purple (P) or white (p)

Genetic TerminologyTrait - any characteristic that can be passed from parent to offspring

Heredity - passing of traits from parent to offspring

Genetics - study of heredity

Gene or Trait Terms Alleles - two forms of a gene (dominant & recessive)

Dominant - stronger of two genes, will hide the recessive trait represented by capital letters (R)

Recessive - gene that shows up less often in a cross;represented by lowercase letters (r)

Genotype - gene combination for a trait (example: RR, Rr, rr)

Phenotype - the physical feature resulting from a genotype (example: red or white)

More Gene Terminology Homozygous (SAME) - genotype combination involving 2 dominant or 2 recessive genes(example: RR or rr); also called pure 

Heterozygous (DIFFERENT) genotype combination of one dominant & one recessive allele(example: Rr); also called hybrid

Types of Genetic Crosses Monohybrid cross - cross involving one trait. example: flower color

Dihybrid cross - cross involving two traits. example: flower color & pea color

Monohybrid Crosses

Punnett Squares • A tool that is used to help solve or predict the inheritance of traits from parents to offspring.

Monohybrid Cross • Trait: Seed Shape•Alleles: R – Round r – Wrinkled•Cross: Round seeds x Wrinkled seeds

RR x rr

What do the peas look like?

Genetic Practice Problems

Cross•A tall plant (TT) with a dwarf plant (tt)

Cross• Tall pea plant (Tt) with another tall pea plant (Tt)

Mendel’s Laws of Inheritance

Law of Dominance •In a cross of parents that are pure for different traits (round or smooth), only one form of the trait will appear in the next generation.•All the offspring will be heterozygous and express only the dominant trait.•RR x rr yields all Rr (round seeds)

Law of Segregation •During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.•Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

Applying the Law of Segregation

Law of Independent Assortment

•Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

•This law can be illustrated using dihybrid crosses.

Dihybrid Crosses •Traits: Seed shape & Seed color•Alleles: R round Y yellow

r wrinkled y green

RrYy x RrYy

RY Ry rY ry RY Ry rY ry

All possible gamete combinations

Parents

Dihybrid Cross RY Ry rY ry

RY

Ry

rY

ry

Dihybrid Cross

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RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 19:3:3:1 phenotypic ratio

RY Ry rY ry

RY

Ry

rY

ry

Incomplete Dominance

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•Hybrids (Rr) have an appearance in between the phenotypes of the two parental varieties.•Example: snapdragons (flower)•red (RR) x white (WW)

RR = red flowerWW= white flower

R

R

W W

Incomplete Dominance

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RW

RW

RW

RW

R

R

W

All RW = pink(heterozygous pink)

W

Codominance •Two alleles are expressed (multiple alleles) in heterozygous individuals.•Example: blood type

1. type A = IAIA or IAi

2. type B = IBIB or IBi

3. type AB= IAIB

4. type O = ii

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Codominance Problem

•Example: homozygous male Type B (IBIB)• x

heterozygous female Type A (IAi)

IAIB IBi

IAIB IBi

1/2 = IAIB

1/2 = IBi IB

IA i

IB

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Another Codominance Problem

• Example: male Type O (ii) x female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

Sex-Linked Traits •Traits (genes) located on the sex chromosomes•Sex chromosomes are X and Y•XX genotype for females•XY genotype for males•Many sex-linked traits carried on X chromosome

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Sex-linked Traits

Sex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Eye color in fruit flies

Pedigree Charts

Female Carriers

Genetic Disorders •Hemophilia – blood does not clot-Queen Victoria

•Sickle cell Anemia- predominantly African Americans - Results in abnormally shaped red blood cells

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•Down Syndrome•Extra chromosome in 21st pair (trisomy)•Mild to severe disabilities•More common in pregnant women over 40-45•Short, stocky, almond shaped eyes, thick tongue and prone to heart defects

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•Klinefelter’s syndrome•Extra sex chromosome (trisomy)•Males (XXY) •Under developed testes, sterile and feminine

•Turner syndrome•1 less chromosome (monosomy)•Females (XO)•Normal childhood but then do not develop sexually, sterile

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•Dyslexia – letters and numbers are backwards